1. Field of the Invention
The present invention relates to a motion vector detection apparatus and a motion vector detection method, and more specifically, the present invention relates to a method for detecting a motion vector between pictures.
2. Description of the Related Art
In recent years, digitized information is widely used in multimedia products. Together with the widespread use of digitized information, a high quality image is desired in the field of video broadcasting. For example, broadcasting media is now shifting from a conventional standard definition (SD) system of 720×480 pixels to a high definition (HD) system of 1,920×1,080 pixels.
As the demand for a high quality image grows in video broadcasting, an amount of digital data is increasing. Accordingly, compression-coding and decoding are required which are more advanced than a conventional method.
To deal with such requirement, International Telecommunication Union Telecommunication Standardization Sector Study Group 16 (ITU-T SG16) and International Organization for Standardization/International Electro-technical Commission, Joint Technical Committee 1/Sub Committee 29/Working Group 11 (ISO/IEC JTC1/SC29/WG11) have been working to standardize coding methods using interpicture prediction, which utilizes the correlation between pictures. Now, H.264/Moving Picture Experts Group-4 Part 10 Advanced Video Coding (H.264/MPEG-4 PART10 (AVC)), among various coding methods, can compress data at highest efficiency. Hereinafter, the H.264/MPEG-4 PART10 (AVC) is simply referred to as “H.264.”
In the case of H.264, the degree of freedom when a user selects a reference picture for detecting a motion vector, becomes relatively higher than in a conventional method. H.264 detects a motion vector by dividing a picture to be coded into the unit of a macroblock or in a smaller unit. Thus, H.264 enables detection of a moving vector in a unit smaller than a conventional method. Accordingly, H.264 can reduce the amount of codes generated in the compression-coding processing.
Japanese Patent Application Laid-Open NO. 2005-184694 discusses a method using H.264, in which a plurality of frame memories are provided and a user can select a reference picture to be used in coding of a picture to be coded from among a plurality of pictures stored in the frame memories.
Conventional coding methods, such as MPEG-1, MPEG-2, and MPEG-4 have a forward prediction function for predicting a following picture according to a previous picture and a backward prediction function for predicting a previous picture according to a following picture.
Here, “predicting a previous picture according to a following picture” refers to prediction of a picture on which coding processing has not been performed, according to a current picture. In the following description, the conventional coding methods, namely, MPEG-1, MPEG-2, and MPEG-4, are collectively referred to as an “MPEG coding method.”
Inmost cases, a picture closer in terms of time, shows relatively higher correlation with the picture to be coded. Therefore, in forward prediction and backward prediction according to the MPEG coding method, an “I picture” or a “P picture,” which exists close to a picture to be coded, are generally used as a reference picture.
However, in the case of a video camera having an MPEG-compliant coder-decoder (codec), it is possible that considerable change occurs between pictures if the video camera is quickly moved during panning and tilting at the time of shooting a moving image, or if a picture is shot immediately after a cut change.
In such a case, even temporally close pictures cannot have a high correlation with each other. Accordingly, it is not useful to utilize motion compensation prediction in such a case.
In order to address this problem, H.264 employs an advanced prediction method. H.264 performs prediction coding on a temporally distant picture as well as on a close picture. If it is expected that coding efficiency can be improved with a temporally distant picture compared to a close picture, H.264 uses the temporally distant picture as a reference picture.
As described above, according to H.264, a user can freely select as reference an input picture that is least different from a coded picture, even if the video camera shooting a moving image is moved quickly or after a cut change has been inserted. Thus, motion compensation prediction can be performed with a high accuracy.
However, if calculation for selecting a picture that is least different from an input picture is performed on all coded pictures, an amount of calculation increases in proportion to the number of reference candidate pictures. Thus, it takes a long time in coding a picture.
Furthermore, in the case of a mobile apparatus such as a video camera, consumption of a battery increases as the calculation load increases. Consequently, time duration in which a picture can be taken becomes short.